Render out of compositor without rendering scene?
Alternative 1



Know that by default the Compositor renders the current scene and that image is used by the RenderLayer Node.  The compositor can use any combination of live Rendered Layers or previously created image files.  The render layer can come from any scene present.  See the nodes options above.

If your Compositor work only requires a few changes and your scene take a long time to render, your work may be more efficient by saving files in PNG format or other more flexible format. Then reuse the images without rendering again.

You may direct the image compositor to read in files from an image sequence on hard disk or similar, and ignore the render layer node.

In the image above the Compositor uses an Image Sequence Node.  The Render layer Node is not used, it has no connections.  A camera exists and non renderable text.  The arrows show the setting for the Compositor.  Please click on the image to see a larger version.

Alternative 2.



You can create a single image plane and map previously created images to it as an image sequence. Perhaps Shadeless for Blender Render and a different setting for Cycles or other. You may need to place a camera and non renderable object such as a non rendered cube

Create the initial set of Image Files.




Render your complex scene as images to a PNG file sequence with some file name sequence such as base name sequence001.  No compositing.  The idea is multiple compositing nodes will be used later.
In a separate scene, for convenience, use the compositor, read in the file sequence001.  Composite these images in the compositor.  
You might put some non renderable text object in the scene such as "See the compositor" as a form of notes.  Otherwise this blender scene might have very few objects.
Repeat step 02 if necessary to achieve your goal.
The assumption is that your compositor nodes are doing more the the Blender VSE can.  VSE and the Compositor are sometimes in Competition.
Some Blender Users may have special file writing needs.